The present invention is directed to an improved unit injector for injecting fuel in an internal combustion engine. More particularly, the invention is directed to an improved unit injector which is capable of producing staged injection of fuel in the internal combustion engine.
Unit injectors, per se, are known. They are characterized by the combination of a pump such as a jerk type pump and a spray nozzle in a single unit, which is adapted to be mounted on the engine, e.g., on the cylinder head. This design eliminates the problems of pressure waves and fuel compressibility in long discharge tubings, but it requires that means be provided on the engine for actuating the injectors. For example, the pump plunger follower can be depressed by a roller at the end of a cam actuated rocker provided on the engine and returned by a follower spring. The unit injector as it is known, however, does not have the capability to produce staged injection, that is, two or more discrete fuel injections during a single stroke of the internal combustion engine.
Thus, an object of the present invention is to provide an improved unit injector which overcomes the aforementioned disadvantage of the known unit injectors. More particularly, an object of the invention is to provide an improved unit injector which is capable of producing a plurality of discrete fuel injections during a single cycle of the internal combustion engine so that staged injection can be accomplished.
The advantages that are achieved by utilizing a staged injection technique are described in the SAE Technical Paper No. 830246. In short, staged injection involving the injection of a small portion of the fuel early in the cycle to act as a pilot for a main fuel charge injected later in the cycle expands the diesel engine cetane limits. As reported in the aforementioned paper, where both pilot and main charges were the same fuel, knocking was eliminated on fuels with cetane numbers as low as 17 at the standard 16:1 compression ratio.
The aforementioned and other objects of the present invention are attained by providing a unit injector according to the invention for injecting fuel in an internal combustion engine. The unit injector comprises a fuel pump and a spray nozzle combined in a single unit adapted to be mounted on the engine with the fuel pump adapted to be driven by a mechanism provided on the engine, and wherein the improvement comprises providing means in said unit injector for producing a plurality of discrete fuel injections during a single cycle of the internal combustion engine.
According to a further feature of the present invention, the unit injector includes means for discharging respective ones of the plurality of discrete fuel injections from different discharge orifices of a plurality of discharge orifices provided in the spray nozzle of the unit injector. The means for discharging respective ones of the plurality of discrete fuel injections from different discharge orifices of the plurality of discharge orifices in the spray nozzle includes a plurality of valves which respectively open the different discharge orifices of the spray nozzle for injection of the plurality of discrete fuel injections.
In the disclosed embodiment, the plurality of valves include first and second needle valves arranged in telescoping relation. First and second biasing means are provided for respectively resiliently biasing the first and second needle valves in positions to close respective one of the different discharge orifices in the spray nozzle. Fuel passage means are provided in the unit injector for delivering fuel from the fuel pump to the spray nozzle. First and second pressure chambers are provided around respective portions of the first and second needle valves. The first and second pressure chambers are in fluid communication with the fuel passage means so that fuel from the fuel passage means can be supplied to the pressure chambers for moving the respective valves against the biases of the first and second biasing means to open the discharge orifices for fuel injection. The magnitudes of the resilient biasing forces of the first and second biasing means are selected so that their forces correspond to different fuel pressures of, for instance, 4000 psi and 2000 psi, respectively. With such an arrangement, only the second or pilot valve will open when fuel is delivered from a second or pilot compression chamber of the fuel pump at the lower pressure of 2000 psi and only the first or main valve will open when the fuel is delivered from a first or main compression chamber of the fuel pump at the higher pressure of 4000 psi.
More particularly, the fuel pump of the unit injector comprises means defining a first relatively large diameter cylinder. A first relatively large diameter piston or plunger is positioned at least partially within the first cylinder and is adapted to be driven along the longitudinal axis of the cylinder by a conventional mechanism provided on the engine for pumping the fuel. An end of the first plunger within the first cylinder defines an end of a first or main fuel compression chamber of the fuel pump within the first cylinder. The end of the first plunger also has a second relatively small diameter cylinder formed therein. A second relatively small diameter plunger extends through the first fuel compression chamber and into the second cylinder for relative movement in the second cylinder as the first plunger is driven by the mechanism. An end of the second plunger within the second cylinder defines an end of a second or pilot fuel compression chamber of the pump within the second cylinder. Fuel inlet and outlet passage means are provided for conveying fuel to and from the first and second fuel compression chambers. The inlet and outlet passage means are arranged so as to be opened and closed by the first plunger during its movement in the first cylinder and relative to the second plunger for producing discrete fuel injections from the first and second fuel compression chambers during a single stroke of the first plunger. The two discrete fuel injections from the first and second fuel compression chambers, respectively, each occur at a predetermined time and are of a predetermined duration depending upon the arrangement of the inlet and outlet passage means.
The outlet passage means for the second or pilot fuel compression chamber includes a bore extending through the second plunger. The second plunger is provided with a flange at an end thereof located outside of the second cylinder. The flange defines an end of the first or main fuel compression chamber. The outlet passage means for the first or main fuel compression chamber includes a bore formed in the flange of the second plunger.
The outlet passage means from each of the first and second compression chambers is in fluid communication with the fluid passage means for delivering fuel from the fuel pump to the spray nozzle. Check valve means are provided in each of the outlet passage means to prevent fuel flow into the fuel compression chambers from the outlet passage means during the injections of fuel from the respective fuel compression chambers of the fuel pump.
These and other objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, one embodiment in accordance with the present invention .